Tiered Replicants & High Availability

Per this discussion[1], it seems that the main purpose of tiered replicants is for high availability.

However, my understanding is that if a historical node crashes, the coordinator will move segments that the crashed historical node was responsible for to a new historical node.

If thats the case, are the replicants just to provide high availability during this period where the affected segments are being spun up on a new node?

Thanks.

[1] https://groups.google.com/forum/#!topic/imply-user-group/6Mghcw3zNpU